AD9518-4A-PCBZ AD [Analog Devices], AD9518-4A-PCBZ Datasheet - Page 30

• Manufacturer Part Number: AD9518-4A-PCBZ
• Description: 6-Output Clock Generator with Integrated 1.6 GHz VCO
• Manufacturer: AD [Analog Devices]

AD9518-4
DIGITAL LOCK DETECT (DLD)
By selecting the proper output through the mux on each pin,
the DLD function can be made available at the LD, STATUS,
and REFMON pins. The DLD circuit indicates a lock when the
time difference of the rising edges at the PFD inputs is less than
a specified value (the lock threshold). The loss of a lock is
indicated when the time difference exceeds a specified value
(the unlock threshold). Note that the unlock threshold is wider
than the lock threshold, which allows some phase error in
excess of the lock window to occur without chattering on the
lock indicator.
The lock detect window timing depends on three settings:
the digital lock detect window bit (Register 0x018[4]), the
antibacklash pulse width setting (Register 0x017[1:0], see Table 2),
and the lock detect counter (Register 0x018[6:5]). A lock is not
indicated until there is a programmable number of consecutive
PFD cycles with a time difference that is less than the lock detect
threshold. The lock detect circuit continues to indicate a lock
until a time difference greater than the unlock threshold occurs
on a single subsequent cycle. For the lock detect to work properly,
the period of the PFD frequency must be greater than the unlock
threshold. The number of consecutive PFD cycles required for
lock is programmable (Register 0x018[6:5]).
Analog Lock Detect (ALD)
The
use at the LD pin. There are two versions of ALD, as follows:
•
•
The analog lock detect function requires an R-C filter to
provide a logic level indicating lock/unlock.
Current Source Digital Lock Detect (DLD)
During the PLL locking sequence, it is normal for the DLD
signal to toggle a number of times before remaining steady
when the PLL is completely locked and stable. There may be
applications where it is desirable to have DLD asserted only
after the PLL is solidly locked. This is made possible by using
the current source lock detect function.
AD9518
N-channel open-drain lock detect. This signal requires a
pull-up resistor to the positive supply, VS. The output is
normally high with short, low-going pulses. Lock is indicated
by the minimum duty cycle of the low-going pulses.
P-channel open-drain lock detect. This signal requires a
pull-down resistor to GND. The output is normally low
with short, high-going pulses. Lock is indicated by the
minimum duty cycle of the high-going pulses.
Figure 35. Example of Analog Lock Detect Filter
provides an ALD function that can be selected for
Using an N-Channel Open-Drain Driver
ALD
AD9518-4
LD
R1
V
S
= 3.3V
R2
C
V
OUT
Rev. B | Page 30 of 64
This function is set when it is selected as the output from the
LD pin control (Register 0x01A[5:0]). The current source lock
detect provides a current of 110 μA when DLD is true, and it
shorts to ground when DLD is false. If a capacitor is connected
to the LD pin, it charges at a rate that is determined by the current
source during the DLD true time but is discharged nearly instantly
when DLD is false. By monitoring the voltage at the LD pin (top
of the capacitor), it is possible to get a logic high level only after
the DLD has been true for a sufficiently long time. Any momentary
DLD false resets the charging. By selecting a properly sized
capacitor, it is possible to delay a lock detect indication until
the PLL is stably locked and the lock detect does not chatter.
The voltage on the capacitor can be sensed by an external
comparator connected to the LD pin. However, there is an
internal LD pin comparator that can be read at the REFMON
pin control (Register 0x01B[4:0]) or the STATUS pin control
(Register 0x017[7:2]) as an active high signal. It is also available
as an active low signal (REFMON, Register 0x01B[4:0] and
STATUS, Register 0x017[7:2]). The internal LD pin comparator
trip point and hysteresis are listed in Table 15.
External VCXO/VCO Clock Input (CLK/ CLK )
CLK is a differential input that can be used as an input to drive
the
up to 2.4 GHz. The pins are internally self-biased, and the input
signal should be ac-coupled via capacitors.
The CLK/ CLK input can be used either as a distribution-only
input (with the PLL off), or as a feedback input for an external
VCO/VCXO using the internal PLL when the internal VCO is
not used. The CLK/ CLK input can be used for frequencies up
to 2.4 GHz.
AD9518
CLK
CLK
clock distribution section. This input can receive
VS
Figure 36. Current Source Digital Lock Detect
COMPARATOR
AD9518-4
Figure 37. CLK Equivalent Input Circuit
LD PIN
110µA
2.5kΩ
5kΩ
5kΩ
DLD
2.5kΩ
LD
REFMON
OR
STATUS
CLOCK INPUT
STAGE
C
V
OUT
Data Sheet